1. Field of the Invention
The present invention generally relates to charged particle beam optical systems and, more particularly, to enhancement of exposures made by projection of a reticle pattern onto a target.
2. Description of the Prior Art
Numerous lithography techniques are known and are in widespread use for manufacture of integrated circuit devices, in particular. Essentially, such lithography processes define potentially very minute areas and shapes on a surface through selective exposure and removal of portions of a layer of resist to expose areas of the surface for further processing by, for example, etching, implantation and/or deposition. Exposure has generally been accomplished in the past by use of radiant energy such as visible wavelength light.
There is a strong incentive in the manufacture of integrated circuits to increase integration density to the greatest possible degree consistent with acceptable manufacturing yields. Device arrays of increased density provide increased performance since signal propagation time is reduced with reduced connection length and capacitance.
To form feature sizes in accordance with design rules incorporating sub-one-tenth micron regimes and below, it has been necessary to perform resist exposures with charged particle beams. Electron beams (e-beams) are favored since particles of the low mass of an electron can be more readily manipulated (e.g. accelerated, focused, guided and the like) at lower power levels preferably using magnetic lenses, correctors, deflectors and the like. Accordingly, while the invention will be discussed below in connection with an electron beam tool, it is to be understood that all such references are intended to be inclusive of tools using ions (e.g. of any element) or electrons. By the same token, the principles discussed herein are also applicable to other charged particle beam devices for purposes other than lithography or semiconductor device manufacture. Thus, references to an e-beam tool should be understood as inclusive of such other charged particle beam devices.
Early electron beam exposure tools were generally of the so-called xe2x80x9cprobe formingxe2x80x9d type and were capable of exposing only a single spot or a relatively small number of pixels at a time. As chip complexity has increased, the throughput of such tools has become generally prohibitive for production of even modest numbers of a moderately complex integrated circuit design of high integration density. Accordingly, electron beam projection tools capable of concurrent exposure of hundreds of thousands to several millions of pixels have been developed.
At the same time, however, problems in projection fidelity and image quality not previously recognized or considered to be of practical importance have become critical to high manufacturing yields of high integration density chips, particularly as minimum feature size has been extended into regimes below one-tenth micron. Many of these problems are inherent in the electron beam optics in regard to proper abutment of sub-fields and aberrations which may be difficult to avoid. Other problems accrue from the complex dynamics of the beam of charged particles, itself, and the interaction of the beam with a target and/or the reticle.
One of the latter type of problems is referred to as the Proximity Effect. This effect derives from the fact that the material to be exposed is generally a resist coating on a target, such as a wafer. Electrons reaching the target wafer through the resist are backscattered with a distribution of trajectories which varies as the cosine of the angle of the backscattering trajectory relative to the incident beam. These backscattered electrons will also cause exposure of the resist.
In broad areas of resist to be exposed, the additional over-exposure will be of little practical effect when the backscattering occurs into regions which are also exposed. However, the exposure caused by backscattered particles contributes to the exposure dose which must be adjusted to avoid xe2x80x9cbloomingxe2x80x9d at the edges of wide areas. This effect and the corresponding potential exposure artifacts are substantially more pronounced in areas of high pattern density (corresponding to closely spaced transparent areas on the reticle) where the spatial frequency of edges is greater and there is a greater likelihood of particles being backscattered into regions which are not intended to be exposed. Since the electron backscatter range is on the order of tens of microns for high energy (e.g. 50-100 keV) electron beams, the net effect of the proximity effect on sub-micron line and space features is an overall background dose boost if the features are located in a portion of the designed field with numerous neighboring features, referred to as high pattern density. Thus compensation appropriate to such regions may require a substantial reduction in direct exposure (e.g. of incident particles before backscattering by the target) dose to prevent overexposure relative to areas of lower pattern density.
However, in extremely large-scale integrated circuits in which a wide variety of circuits of differing functionalities are formed, it is to be expected that the pattern density will also vary widely. Thus the net result of the proximity effect, when the dose is adjusted to obtain suitable exposure of high pattern density areas, is to substantially underexpose areas of low pattern density (corresponding to relatively isolated transparent features on the reticle).
Another effect that potentially limits projection fidelity is referred to as the Local Coulomb Effect (LCE). Equally charged particles are mutually repelled by each other and this effect is cumulative, the net defocus varies with the local density of equally charged particles in the beam. Since the beam is patterned by the reticle in electron beam projection tools, in portions of each sub-field that correspond to a higher percentage of exposed area, electrons will experience a greater degree of mutual repulsion in the beam. This mutual repulsion is manifested in the manner of a plurality of small local (space charge) lenses. The complexity and magnitude of the effect and corresponding exposure artifacts are increased in charged particle beam projection systems because of the relatively large area (e.g. xc2xc mm to 1 mm square) sub-field exposure area and the complexity of the pattern therein.
Higher percentages of exposed area correspond to both large exposed areas and areas of high pattern density. The net effect is one of local defocussing the beam and variation of the focal plane with percentage of exposed area in the pattern, causing variable blurring of portions of the image. In contrast with the proximity effect which locally affects exposure dose, the local Coulomb effect locally affects focus.
Three types of reticle mask are now in general use: a stencil mask, a so-called scattering mask and a stencil/scattering mask which combines operating principles of both the stencil mask and the scattering mask. The latter two of these types of reticle or mask develop significant populations of scattered particles and rely on use of a contrast aperture/mask to block particles having trajectories which do not correspond to the intended exposure pattern defined by the mask or reticle. The only scattering produced by a stencil mask is a result of non-vertical slope of stencil walls.
Specifically, in a stencil mask, a metallic foil of a thickness sufficient to absorb electrons of the energy level chosen for the exposure, is locally perforated. Electrons impinging on the remaining metallic portions of the mask are thus principally absorbed. Electrons passing through apertures are not absorbed or scattered. This type of mask is not generally favored since the high degree of absorption of charged particles and corresponding current causes heating and mask distortion.
In a so-called scattering mask, the mask membrane is very much thinner than in the stencil mask and generally formed of silicon nitride. The pattern is formed as a pattern of material of higher atomic number such as a metal which is much more strongly scattering of electrons than the very thin layer of silicon nitride. contrast is developed by use of a contrast aperture which differentially intercepts strongly and weakly scattered electrons.
The third type of reticle or mask is constructed similarly to the stencil mask, having apertures in a membrane and without strongly scattering material deposited thereon, but operates much in the manner of a scattering mask. To reduce particle absorption and heating of the reticle, the thickness of the mask membrane is adjusted in accordance with the particle energy so that absorption is avoided, but transmitted electrons are scattered significantly due to interactions with the mask material. Electrons passing through apertures are not absorbed or scattered. Again, as with scattering masks, the pattern is developed by a contrast aperture which intercepts the predominant portion of scattered particles.
It can be appreciated that each of these types of masks or reticles is intended to produce either full exposure or no (significant) exposure. Therefore, efforts to compensate for the proximity effect such as edge or feature size adjustment (by xe2x80x9ctrimmingxe2x80x9d of the reticle features), multiple exposures (e.g. illuminating the reticle with differently patterned beams), xe2x80x9cghostxe2x80x9d corrections (using scattered electrons to provide background dose boost), dose modulation (standard in probe-forming systems but inapplicable to projection systems) and the like greatly increase complexity of the exposure process and potentially compromise the success of the process because they inherently involve the accuracy of respective portions of the reticle, may not be controllable enough at a local level, reduce system throughput and/or require registration of plural exposures. Compensation for local Coulomb effect would require regulation of percentage area of exposure in the circuit design which is not at all practical and, even if possible, would unacceptably constrain integrated circuit layout designers. Accordingly, it can be seen that, at the present state of the art, addressing the proximity and local Coulomb effects necessitates undesirable economic tradeoffs.
It is therefore an object of the present invention to provide a technique for simultaneous compensation of proximity effect and local Coulomb effect without imposing restrictions or complications on integrated circuit layout design.
It is another object of the present invention to provide a novel type of electron beam projection reticle having improved performance, particularly in regard to reduction of proximity effect and local Coulomb effect.
It is a further object of the present invention to provide an electron beam projection reticle capable of three independent levels of exposure dose which can be patterned without design restrictions.
It is yet another object of the present invention to provide significantly improved fidelity of charged particle beam projection exposures with elimination or substantial reduction of dose variation and local defocussing artifacts.
In order to accomplish these and other objects of the invention, a charged particle projection reticle is provided including an weakly scattering apertured membrane and strongly scattering deposits between the apertures in the weakly scattering membrane that, in combination with a contrast aperture provides at least three levels of exposure dose; simultaneously including adjustable proximity effect correction boost and local Coulomb effect correction.
In accordance with another aspect of the invention, a method of performing charged particle beam projection lithography and method of semiconductor device manufacture are provided including the steps of distributing strongly scattering deposits on a weakly scattering reticle membrane and projecting a beam of charged particles through said reticle and a contrast aperture onto a target whereby at least three levels of charged particle flux are developed to approximately compensate proximity effect and local Coulomb effect simultaneously.